A COMPARISON OF TWO METHODS FOR TESTING THE RESISTANCE OF FRESH SCC TO SEGREGATION

Transcription

1 A COMPARISON OF TWO METHODS FOR TESTING THE RESISTANCE OF FRESH SCC TO SEGREGATION L. Tang 1,2, J.-E. Lindqvst 1, C. Ewertson 1 and D. Boubitsas 1 1 SP Technical Research Institute of Sweden, Borås, Sweden 2 Chalmers University of Technology, Gothenburg, Sweden Abstract This paper presents experimental results from a comparison study of two methods, the sieve stability test and the J-ring test, for testing the resistance of fresh self-compacting concrete (SCC) to segregation. The sieve stability test is based on the measurement of the sieved portion of the sample passing through a 5 mm sieve, while the J-ring test is based on the difference between two measurements of the blocking step. Different mixes of fresh SCC with natural, blended natural and crushed, and crushed aggregates were tested in parallel using both the test methods. Concrete were cast in plastic pipes with 2 m height using some of representative mixes with crushed aggregates. Specimens from different heights were taken for testing compressive strength and chloride migration coefficient. The results show that there is a fairly good correlation in the measurement results from the two methods for the SCC with natural aggregates, but not for that with crushed aggregates. The relationships between the fresh and hardened properties of SCC as well as some discrepancies between the two methods in the test results are discussed. 1. INTRODUCTION It is a common understanding that the workability of fresh self-compacting concrete (SCC) should basically include three key properties: filling ability, passing ability and resistance to segregation. In the 5 th European FP project Testing-SCC (GROWTH, GRD ), various methods for testing fresh properties of SCC were evaluated [1]. Based on the evaluation results, the sieved stability test was recommended for testing the resistance of fresh SCC to segregation [2]. This method measures the portion of the fresh SCC sample passing through a 5 mm sieve. If the SCC has poor resistance to segregation, the paste or mortar can easily pass the sieve. Therefore the sieved portion indicates whether the SCC is stable or not. The method needs the sample to stand still for 15 minutes, which is relatively long for the fresh concrete before casting, because the fresh properties may have changed after this 15 minutes standing. On the other hand, the J-ring test measures both the passing ability and the filling ability. It is very suitable for field application. This method was, therefore, adopted as the Nordtest 111

2 standard method (NT BUILD 507) for quality control of fresh SCC [3]. Since the J-ring test is very sensitive to the aggregate content, it can also be used to detect the difference in blocking step due to different aggregate content caused by segregation. This paper presents some results from a comparison study. 2. BRIEF DESCRIPTION OF TEST METHODS 2.1 Sieve stability test This method was developed in France [4] as a quantitative measure of SCC s resistance to segregation. About 10 litres fresh SCC sample is taken into a 10 litre plastic bucket. The sample is covered with a lid and stands still for 15 minutes. After inspection for water bleeding, about 4.8 ± 0.2 kg sample is poured from a height of 50 cm onto the central part of a perforated plate sieve with square holes of 5 mm. The test result is expressed by the sieved portion (the mass percentage of the sample passing through the sieve). The detailed description of the method has been given in [2]. 2.2 J-ring test The principle of the J-ring test may have originated in Japan but no references are known. The original form of the J-ring test has been developed further at the University of Paisley in Scotland [5]. The equipment consists of a ring of Ø300 mm placed on 16 pieces of Ø18 mm rebars with a height of 140 mm, combined to the slump flow test using Abram s cone. For an ordinary test, 6 litres fresh SCC sample is used. The test procedure is similar to the slump flow test, with the J-ring around the Abram cone. After spreading the sample, the spread diameter is measured in the same way as in the slump flow test. The blocking step is calculated from the difference between the sample height at the centre and the average sample height outside the J-ring. The detailed description of the method has been given in [2,3]. When the J-ring test is used for estimating the resistance to segregation, 12 litres fresh SCC is taken into a plastic bucket of 300 mm inside diameter. After standing still for 2 minutes, the ordinary test procedures is performed twice using the top half and the bottom half respectively of the 12 litres sample in the bucket. The change in the blocking step between the two measurements is calculated using the following equation [3]: δ ( B B ) J2 J1 BJ = BJ 100 where, B J1 and B J2 denote the blocking step from the first and the second measurements, respectively, and B is the mean value of the two measurements. J 3. EXPERIMENTAL In this study, different mixes of fresh SCC were tested in parallel using both the J-ring and the sieve stability tests. These mixes include SCC with natural aggregates, with combination of natural fine and crushed coarse aggregates and with fully crushed aggregates. The test results are shown in Table 1. (1) 112

3 Table 1: Results from the J-ring and sieve stability tests J-ring spread [mm] J-ring T50 [sec] Blocking step [mm] Change in blocking step Sieved portion Mix No. Test 1 Test 2 Test 1 Test 2 Test 1 Test 2 [%] [%] N N N N N n.d N N N8 (D25) N9 (D25) HC HC HC FC FC > FC FC FC FC FC FC FC N = Natural fine 0-8 mm + Natural gravel 8-16 mm ( mm in N8 and N9); HC = Natural fine 0-8 mm + Crushed aggregate 8-16 mm ; FC = Crushed fine 0-2 mm + Crushed aggregates 2-4 (or 2-5), 4-8 (or 5-8) and 8-16 mm. Due to the financial limitation, only the last four mixes with crushed aggregates, i.e. mixes FC6 to FC9 (corresponding to Mix A to Mix D in [6], where the detailed mix proportions are given), were cast into plastic pipes of 100 mm in diameter and 2 m in length, which was vertically placed in the laboratory during casting and the first day curing. After casting, each pipe s openning was sealed with plastic sheet to keep moist curing condition. After 3 weeks curing, the test specimens (100 mm thick for the compressive strength test according to SS EN , 50 mm thick for the rapid chloride migration test according to NT BUILD 492, and 10 mm for visual examination and image analysis) were cut from each pipe at three different heights, i.e. about 0-40 cm, cm and cm. Two specimens at each height were taken. The plastic pipe around the specimen was removed after cutting, and the concrete specimens were stored in the laboratory before testing at 28 days. The test results from these four mixes (FC6 to FC9) of hardened concrete are shown in Table

4 Table 2: Results from four mixes of hardened concrete Height Compressive strength at 28 d [N/mm 2 ] (average of 2 Ø specimens) Chloride migration coefficient at 28 d [ m 2 /s] (average of 2 specimens) [cm] FC6 FC7 FC8 FC9 FC6 FC7 FC8 FC Mean COMPARISONS AND DISCUSSIONS Figure 2 shows the comparison between the results from the sieve stability test and the J- ring test. Sieved portion [%] N1-7 N8&9 HC1-3 FC1-5 FC Change in blocking step [%] Figure 2: Comparison of results from the sieve stability test and the J-ring test For the SCC with natural aggregates, there exists a fairly good correlation in the measurement results from these two methods, especial when the particle size is <= 16 mm. It seems that, for this type of SCC, the sieve stability test always results in a certain sieved portion of 10-20%, even thought there is no change in blocking step (e.g. mixes N1 and N2). It is to a great extend an indication that the sieve stability is more sensitive to the paste separation or water bleeding, which can easily pass the sieve even when the separation layer is a few millimetre thick. This small separation may not significantly change the macro distributions of aggregates in the fresh SCC, resulting in no change in blocking step. For the SCC with crushed aggregates, however, no clear correlation between these two methods could be found. The sieved portion from the sieve stability test seems not over 20%, while the change in blocking step can be very significant. It seems that the sieve stability test is insensitive to the SCC with crushed aggregates, probably owing to its less tendency of paste separation or water bleeding. The relative changes in strength and chloride transport property are shown in Figures 3 and 4, respectively. It can be seen that the changes in compressive strength are limited to ±5%, 114

5 even for the concrete mix FC9, which revealed a large change in blocking step (133%, see Table 1). The changes in chloride transport property are, however, very significant for this mix (about 40% change from the bottom 0-40 cm to the top cm, as shown in Figure 4), indicating a segregation that results in an inhomogeneous transport property. Height cm FC6 Height cm FC7 FC8 FC9 Height 0-40 cm -6% -4% -2% 0% 2% 4% 6% Change relative to the mean value Figure 3: Relative change in compressive strength at different heights Height cm FC6 Height cm FC7 FC8 FC9 Height 0-40 cm -30% -20% -10% 0% 10% 20% 30% Change relative to the mean value Figure 4: Relative change in chloride migration coefficient at different heights Figure 5 shows some photographs of concrete s corss-section taken from different heights. Although concrete mix FC9 revealed less coarse aggregate, there is no visual difference between the images taken from different heights. Further image analysis may be needed to identify acutual segregated components in mix FC9. 115

6 Figure 5: Cross-sectional images of mixes FC7 (left), FC8 (middle) and FC9 (right), taken from a height of 0-40 cm (lower), cm (middle) and cm (upper) 5. CONCLUDING REMARKS For the SCC with natural aggregates, there is a fairly good correlation in the measurement results from the sieve stability test and the J-ring test, while for the SCC with crushed aggregates no clear correlation between these two methods could be found. The sieve stability test is more sensitive to paste separation or water bleeding, while the J-ring test seems more sensitive to aggregate segregation. Segregation may result in more significant change in chloride transport property than in compressive strength. REFERENCES [1] Testing-SCC WP6 Report, Evaluation of Preci-sions of Test Methods for Self-Compacting Concrete, EU Project (5th FP GROWTH) Measurement of Properties of Fresh Self-Compacting Concrete, GRD /G6RD-CT , (2004). [2] Testing-SCC, Guidelines for testing fresh self-compacting concrete, EU Project (5th FP GROWTH) Measurement of Properties of Fresh Self-Compacting Concrete, GRD /G6RD-CT , (2005). [3] Nordtest NT BUILD 507, Concrete, mortar and cement based repair materials: Quality control of fresh self-compacting concrete Workability, air content, density and casting of test specimens, Nordic Innovation Centre, Oslo, (2006). [4] Association Française de Génie Civil, Bétons Auto-Plaçants Recommandations provisoires, Documents scientifiques et techniques, July 2000, 63 p. [5] Bartos, P.J.M., Sonebi, M. and Tamimi (editors), A.K., Workability and Rheology of Fresh Concrete: Compendium of Tests, Report of RILEM Technical Committee TC 145-WSM, Workability of Special Concrete Mixes, RILEM Publications S.A.R.L., Cachan Cedex, France, [6] Tang, L., Åkesson, U., Schouenborg, B., Ewertson, C. and Boubitsas, D., A study of proportioning techniques for scc with crushed aggregates, to be presented at SCC 2007 Conference, 3-5sept., Ghent, Belgium, (2007). 116